Exploring Focal Lengths
Changing your focal length can do much more than simply give you a wider or more telephoto shot
Depth of field is the amount of sharpness from foreground to background within a photograph. While it shares the idea of depth with perspective, the two photo controls have nothing else to do with each other. Depth of field affects only sharpness in depth. Perspective deals with size changes in depth.
Depth of field is affected by three things: ƒ-stop (smaller apertures like ƒ/16 give more depth of field and large apertures like ƒ/2.8 do the opposite); focusing distance (when you're close, depth of field is shallow and when you're at a distance, it's deep); and, of course, the subject of the moment, focal length.
The shorter the focal length of a lens, the greater the apparent depth of field. Conversely, as you increase the focal length of a lens, apparent depth of field decreases. This can be a significant effect when making a photograph. If you want your background to be more clearly seen behind your subject, you might change to a wider-angle focal length. If you want that background to be less dominant and somewhat out of focus, you could change to a longer focal length.
Landscape photographers often shoot with a wide-angle lens to gain more of a feeling of depth (from perspective) and sharpness (from more depth of field). Portrait photographers typically do just the opposite—shoot with a telephoto lens to make the subject stand out from the background due to less depth of field.
All of these work together, of course: ƒ-stop, focal length and distance. Your least depth of field comes from a wide aperture used on a telephoto lens at a close distance. The most depth of field comes from a small aperture used on a wide-angle lens at a distance. The chart above shows this relationship.
An extreme example of sharpness control is selective focus and hyperfocal distance. Selective focus allows you to precisely define focus on a subject (or part of a subject) and make everything else out of focus. This is done by using a wide aperture, a telephoto lens and getting close to your subject.
If you want to learn to use your lenses more fully and to explore the possibilities of perspective and depth of field, here's an assignment for you. Put a zoom lens on your camera and go out to photograph with no additional gear. Find a fun location and then spend a few hours photographing.
Your selected aperture, lens focal length and physical distance from the subject all combine to effect depth of field. Making some practice exposures and comparing the results will help you master the creative options that various combinations will give you.
Here's the special part of the assignment: Alternate pictures from the widest setting of your zoom to the most telephoto. In other words, take pictures at the widest setting, then immediately zoom to the most telephoto. Now find a new photo without touching the zoom. Next, zoom out to the widest setting and find an interesting photo without touching the zoom. Keep doing this, back and forth, for at least 30 to 40 photos. Compare the results, and you'll see how much more creative control you have when you master these concepts.
Rob Sheppard's book, the Kodak Guide to Digital Photography, covers all aspects of working with a digital camera, including lenses.